How do I tell my boyfriend that's in jail I kissed another man?

he is in jail for a pi that violated his probabtion I tried to stop him but he just kept throwing me down now that he is in there we fight and he sends me really rude letters and I got drunk and made out with another guy I'm not sure how to tell him I don't want to hurt him but I'm not even sure if I want to be with him now what should I do?